Conventionally, when performing an image forming operation on an electronic copying machine, etc., an operator must initially select a paper supply cassette storing papers in desired size among multiple paper supply units through operations of the switches provided on a control panel. The operator then inputs the desired number of copies of an image into an image forming apparatus through ten keys on the control panel. Following the operations, the operator turns ON a copying button or a print key. This causes the image forming apparatus to be supplied papers from the selected paper supply cassette and an original document image to be formed on the papers. This operation is successively repeated for the desired number of papers that is input through the ten keys.
On such an electronic copying machine, etc. there may be a case where papers in the selected paper supply cassette are completely exhausted before completing the copying operation for the desired number of papers that is input through the ten keys. In such a case, if another paper supply cassette storing the same size papers is available, it is possible to succeed the image forming operation and complete the copying of the image for the desired number of papers by switching to the paper supply from that paper supply cassette. However, if no paper supply cassette storing the same size papers is available, there was such a problem that a message "PLEASE FEED PAPER" is displayed, the successive image forming operation was stopped and the desired number of papers with the image formed could not be obtained as the subsequent image forming operation was not carried out.
Therefore, on a complex image forming apparatus with a facsimile unit added to an electronic copying machine for receiving image data composed of digital signals transmitted through data communication, there was such a problem that if fixed size papers were exhausted during the night time data communication, etc., it became impossible to receive data due to the exhaustion of papers provided for the image forming operation.
As described above, there was such a problem that if papers in the paper supply cassette were exhausted and no other paper supply cassette storing the same size papers were available during the successive image forming operation, the operation couldn't be successively carried out but was stopped once and it became necessary, after feeding papers in desired size, to set successive image forming conditions and resume the image forming operation and also, on an apparatus with data communication function provided, the function was stopped.